Resistance and Phasors?

Hi Im a Mechanical engineer but Im taking a module in electrical, which isnt my strong point. Please can someone explain the differnce between bilateral and unilateral resistance and what is a good website to help me understand phasors and sinusoidal quantities.
Thanks x

You can define a resistor as a device in which voltage and current are related to each other at any instant. The relationship is called the characteristic of the resistor.
By this definition, an ohmic resistor, a diode or a voltage source can all be considered resistors.
A bilateral resistor is one whose characteristic curve is symmetrical in relation to the origin. An ohmic resistor is bilateral, a diode is not.